Electrolytic production of chlorine and caustic soda (sodium hydroxide) began in the late 1800's when the industrial revolution required an efficient source of these materials. Production advances in the 1900's increased output, reduced unit costs and improved quality. Many industrial activities, such as the making of polyvinylchloride, paper, aluminum and textiles, depend on the properties of chlorine and caustic soda to obtain quality products. Chlorine and caustic soda are produced by the electrolysis of salt (sodium chloride). Different types of electrolytic cells are used commercially, the most common being diaphragm cells. All work on the principle of passing electrical energy through a brine solution to generate chlorine gas at an anode and hydrogen gas, with caustic soda, at a cathode. In the case of diaphragm cells, asbestos or polymeric diaphragm(s) serve to separate the anode(s) and cathode(s) within the cell. Both the brine solution and products produced are very corrosive and as such, the materials used in constructing electrolytic cells are often determined by their expected lifetimes. The diaphragms generally last about one year, requiring replacement. The need to replace components of the diaphragm cells necessitates a design which provides access to these components. A design quite common in the industry is one wherein the anodes, cathodes and brine solution are housed in a receptacle, typically comprised of concrete, over which a cover or cell head comprised of fiber glass reinforced polyester is positioned to provide a liquid-tight and gas-tight cavity for the anodes and cathodes. A liquid-tight seal between the cell head and the concrete base is required in that the brine solution is typically maintained at a level above the top of the concrete receptacle so as to cover the anodes and cathodes with brine solution. The cell head must provide a gas-tight seal over the anodes and cathodes so as to prevent the loss of the chlorine and hydrogen gas generated.
Cell heads comprised of fiber glass reinforced polyester (FRP cell heads) have provided good service; however, improvements are desired. Due to the corrosive nature of the electrolytic cell environment, it is necessary to reline the FRP cell heads periodically and eventually replace the cell head. The fiber reinforcement tends to provide a "wick" for the corrosive material such as chlorine and caustic soda, allowing the corrosive material to penetrate the surface causing damage which cannot be repaired. In that a number of electrolyte cells are typically operated in series within a chlor/alkali plant, relining and replacement is expensive. A more durable cell head is desired.
The FRP cell heads are also difficult to manufacture, requiring a significant amount of manual labor in laying up the fiber glass reinforcement and applying the resin matrix. A cell head made by a more efficient method is also desired.